The present invention relates to a process for preparing pulverized gels of water-soluble vinyl polymers, and more particularly to a process for preparing particulate water-soluble vinyl polymers without deteriorating the quality of the polymers, without lowering the molecular weight and providing decreased sticking of polymer particles to each other.
Water-soluble acrylic polymers, particularly acrylamide homopolymer, copolymers of a major amount of acrylamide and other copolymerizable monomers, or alkali hydrolysis products of these acrylamide polymers have been widely utilized as sizing agent, viscosity builder, soil improver, waste water treating agent, agent for recovering crude oil, etc. These watersoluble acrylamide polymers have been prepared by bulk polymerization, suspension polymerization, emulsion polymerization or solution polymerization. In general, polymerization of aqueous solutions of monomers has been mainly adopted, since polymers having a high molecular weight are obtained.
For obtaining polymers having a very high molecular weight and moreover having a good water solubility by means of the aqueous solution polymerization method, it is necessary to prevent crosslinking reactions during polymerization as much as possible. In this respect, it is desirable to conduct the preparation of polymers under relatively mild conditions, e.g. conducting polymerization with relatively low concentration of a monomer.
In recent years, due to economic considerations such as transportation costs and storage costs, water-soluble polymers in the form of powder are demanded rather than in the form of an aqueous solution. An aqueous solution polymerization at a low monomer concentration has the defect that the obtained polymers must be dried to evaporate a large quantity of water upon powdering, thus resulting in increase of utility cost for powdering.
Also, partially hydrolyzed acrylamide polymers have been usually prepared by partially hydrolyzing acrylamide polymer gels by using alkaline materials or the like. In that case, too, use of polymer gels having a relatively low concentration of polymers is desired, since it is necessary to make the alkaline material penetrate into the inside of the gels so as to uniformly perform partial hydrolysis. On the other hand, use of the polymer gels having a low concentration has the disadvantages that utility cost for evaporating water is increased and that the polymer gel particles adhere to each other to form agglomerates and the agglomeration remarkably lowers the drying efficiency in the drying of the particles.
In order to eliminate the above-mentioned defects, a study has been made to provide a process for practicing the polymerization at a monomer concentration as high as possible to provide a reduction in the utility cost at the powdering stage.
Vinyl monomers such as acrylamide and acrylic acid show a marked tendency to crosslink and to form a three dimensional structure. Accordingly, in practice, in order to prevent crosslinking from occurring, the polymerization has inevitably been conducted under mild conditions, especially in a relatively low monomer concentration, for example, in a concentration of at most 20 to 30% by weight for anionic or non-ionic monomers.
The polymerization products obtained by subjecting an aqueous monomer solution containing acrylamide alone or containing mainly acrylamide in a low concentration as mentioned above to the polymerization are in the form of a hard or elastic gel having no fluidity. Therefore, when water is removed from the obtained massive or sheet-like polymer gel without mechanically breaking it into pieces, the gel must be allowed to stand at high temperatures for a very long period of time. As a result, the molecular weight is lowered and crosslinking accompanied by heating for drying takes place, thus resulting in marked lowering of the commercial value of the products. Accordingly, there has generally been adopted a process wherein after roughly pulverizing the massive or sheet-like polymer gel to form coarse particles by a mechanical means, water is removed from the particles by drying with heat. In a widely adopted process of drying, a polymer gel obtained by polymerization is formed into a strand-like form by an extruder like a meat grinder, and then dried by heating. However, the use of the extruder is not always satisfactory, particularly when the polymer is very hard, because the friction against the wall surface of the extruder is large, thus not only the machine efficiency is decreased, but also the polymer gel itself is subject to deterioration due to frictional heat or mechanical force, thereby resulting in lowering of the molecular weight due to severance of the polymer molecule.
In order to reduce the frictional resistance of machines, it is proposed to use a lubricant such as polyethylene glycol or non-ionic surfactants upon extruding the polymer gel. However, a large amount of the lubricant must be used for obtaining a satisfactory effect, and it rather brings about undesirable results such as decrease in the purity, increase in sliminess of polymer powder and lowering in free flowability of polymer powder.
For such reasons, it is important to develop a process for pulverizing polymer gels without suffering deterioration due to frictional heat and mechanical force and without lowering the molecular weight.
It is an object of the present invention to solve the above-mentioned problems such as deterioration of polymer gels caused by frictional heat and mechanical force upon breaking or pulverizing the gels, and decrease in purity and flowability of polymers and increase in sliminess of powder which occur when a lubricant is used in pulverizing the gels.
A further object of the invention is to provide a process for easily pulverizing aqueous gels of watersoluble vinyl polymers without deteriorating the polymers and lowering the molecular weight.
A still further object of the invention is to provide a process for preparing finely divided gels of water-soluble vinyl polymers, without, allowing the gels to stick to each other and to an apparatus for performing the process, which gels can be easily dried to give dry powders without causing undesirable crosslinking of the polymers.
These and other objects of the present invention will become apparent from the following description.